The Effect of Current Speed on the Distribution of the Larvae of the Blackflies, Simulium variegatum (Mg.) and Simulium monticola Fried. (Diptera)

Field and laboratory studies on the water-velocity preferences of the larvae of Simulium monticola Fried, and S. variegatum (Mg.) show that both species occur in velocities of 0·5–2·5 m./sec. but aggregate in the velocity range 1·0–2·0 m./sec. The velocity range that the larvae of different species of Simulium can withstand indicates that current speed plays an important part in governing the distribution of blackfly larvae, and perhaps isolating different species along watercourses.